1. Field of the Invention
This invention relates in general to die casting of small parts, and in particular to die casting of small parts made of materials that expand when transition lug from the liquid to the solid state, such as bismuth alloys.
2. Description of the Prior Art
Die casting of small parts from materials such as lead, zinc and tin is well known in the art. Equipment and methods for casting such materials are well known and in widespread use.
There are several types of traditional die casting machines, with hot-chamber machines being popular for die casting small parts. Generally, the die comprises two die halves, a stationary die half and movable die half. Either one or both of the die halves have cavities located therein, which, when the die halves are in the closed position, define the shape of the cast part. To cast a part, the die halves are locked in the closed position, and the molten material is injected into the cavities. After a cooling period, the die halves are separated, and the part is ejected from the die. In order to have proper ejection, it is desirable for the parts to partially stick to the movable die half. This is so because only the movable die half usually has ejection pins. Thus, if parts stick to the stationary die half, they would have to be manually removed, thus preventing the automation of the casting process.
Standard die casting equipment and methods are well suited for use in conjunction with standard die casting materials. However, when standard equipment and methods are used with materials that expand in transitioning between the liquid state and solid state, the standard equipment has been found to be far from adequate. The parts being die cast stick to the wrong side of the die and do not properly release from the die, thus making the casting of parts very inefficient, if not impossible.
The need to die cast small parts out of a material that expands during solidification has only recently become a concern. The recent introduction of shotgun shells comprising pellets, or shot, made from bismuth alloys has begun the search for a technique for forming shot out of bismuth alloys. Bismuth, however, expands when transitioning from the liquid to the solid state, and alloys comprising bismuth generally have the same tendency. Experimentation has revealed that standard die casting equipment and methods do not work with materials that expand upon solidification, including bismuth alloys.
The need exists for an apparatus and process for efficiently die casting small parts from materials that expand when transitioning from the liquid to the solid state.
It is the general object of the invention to provide an apparatus and a process for efficiently die casting small parts from materials that expand when transitioning from the liquid to the solid state.
The present invention is for a die for die casting substantially spherical objects of a material that expands when transitioning from the liquid to the solid state, and an associated method of casting such parts. The die comprises a first and second die halves, each die half having a plurality of cavities that, when the die halves are in a closed position, align with the cavities in the other die half, thus defining a plurality of voids that define the objects to be formed. Each cavity is aspherical in that the slope of the cavity is never perpendicular to the front surface of the die insert, thus facilitating the removal of the objects from the die. A spacing between the front surfaces of the die halves during the injection step of the cycle helps to increase the number of parts that properly release from the die.
The above as well as additional objects, features, and advantages will become apparent in the following description.